This invention relates to a novel semipermeable membrane.
Semipermeable membranes are now used in a wide variety of fields such as waste-water treatment, desaltnation of sea water, food industries and medicinal industries for the purpose of separating a solute from a solution, concentrating a solution or recovering the solvent from a solution. Because of their effectiveness in saving energy and resources, semipermeable membranes will be used more and more in the future.
In the separation technique using semipermeable membranes, reverse osmosis, ultrafiltration or dialysis is generally adopted according to the type of the solution to be treated and the object of the treatment. Thus, the optimum properties of semipermeable membranes also vary depending on the type of the solution to be treated and the object of the treatment. However, the following properties are generally commonly required for semipermeable membranes: a high selectivity in separation of a solute from a fluid (ability of rejecting the passage of the solute through the membrane); a high flux rate (rate of the flux of the solvent through the membrane); a high resistance to heat and chemicals such as an alkali, acid and oxidizing agent; and a high mechanical strength.
Nowadays a variety of semipermeable membranes are commercially available but these membranes fail to completely satisfy the above-described requirements. Therefore, known semipermeable membranes suffer from various drawbacks in practice. For example, a membrane made of acetyl cellulose, which is the most common semipermeable membrane, lacks in chemical and thermal stability and is also susceptible to biodegradation. Membranes composed of a polyamide or polyacrylonitrile are defective in stability to heat and strong alkali and resistance to oxidizing agent such as chlorine gas. Membranes formed of polyvinyl chloride, polyvinyl acetate or polyvinyl carbonate have a defect that the step of forming membranes encounters with considerable difficulty and the resulting membranes have not sufficiently high semipermeability. Among currently available ultrafiltration membranes, polysulfone-type membranes are known to have the highest quality. They are, however, poor in chemical resistance and, moreover, their selectivity is not satisfactory.